This invention relates generally to hearing aids and, more particularly, to hearing aid control circuits for enhancing the intelligibility of speech signals corrupted by low-frequency noise.
Hearing aids presently available are miniaturized so as to fit into the ear canal or over the ear, the main motivation for which is to make the aid unobstrusive. This stems from the myth that a hearing impairment is a sign of decrepitude in an elderly person, or a handicap for a young person. A similar myth existed a generation ago in respect of the wearing of eyeglasses, but this image has now been removed and there no longer is any stigma attached to wearing them.
Because of the flaws in presently available hearing aids many of the millions of hearing-impaired persons, even after having been fitted for hearing aids and paid their not insignificant cost, do not use them. While the art of hearing aids is generally well known and well understood, hearing aid users continue to suffer from the fact that most hearing aids are non-discriminating and therefore equally amplify audio information and background noise. As reported in Hearing Instruments, Vol. 38, No. 8, 1987, pp. 8-16, the most common complaint reported by hearing instrument wearers is that speech was difficult to understand under noisy and reverberant conditions. Studies have shown that listening conditions which are just mildly disruptive for normalhearing persons have serious consequences for understanding speech for hearing-impaired subjects. A normal hearing person in quiet, because the speech is rich in redundant cues, can tolerate a substantial amount of degradation of the speech signal before it becomes unintelligible. However, the set of cues available, especially the crucial high frequency cues, are substantially reduced for the typical hearing-impaired listener, and in noisy, reverberant environments the reduction in available speech cues usually reduces the remaining low frequency information, resulting in further degradation of the primary signal. Additionally, currently available aids cause discomfort to the user when a sudden loud noise occurs, their limited range makes them very ineffective for use in theatres and conference halls, and generally have poor tone quality. Furthermore, the adjustment controls on the hearing aid not only are very small but because of their location on the device (which must be in place in the ear canal or over the ear during adjustment) are very difficult to manipulate.
These flaws of available hearing aids are directly attributable to their miniature size which results in components that are too small to meet the fundamental objective of an effective hearing aid that for normal sounds the acoustic output should be a clear but amplified copy of the input signal. The microphone usually used as the transducer for converting an acoustic input to an electric signal is extremely small and because of its location on the hearing aid device is quite directive (but not necessarily sufficiently or in the right direction). Similarly, the powerhandling capability and the dynamic range of the miniature speaker used in the present hearing aids are extremely limited, factors which contribute to the above-outlined flaws.
Accordingly, there is a need for a hearing aid that can be relatively unobstrusively coupled to the ear which has more power-handling capability and a wider dynamic range than the miniature speaker used in available hearing aids.
Additionally, it is desirable that a hearing aid have the capability of using a variety of superior microphones each tailored for a particular purpose: for example, a built-in microphone for convenience; a plug-in directional microphone to aid in noise control; a plug-in power-enhanced microphone for use in theatres and conference rooms; a plug-in or induction coil telephone pickup.
Moreover, the frequency response of the hearing aid desirably is adjustable to compensate for the frequency losses of the hearing-impaired user, and to enable the user to listen to speech in the midst of ambient background noise.